Spiraling energy costs have renewed interest recently in the recovery of energy from a variety of energy sources. In particular, the recovery of thermal energy from low temperature waste heat streams and low-to-moderate temperature natural heat sources has received much study. For example, work has been done recently on the recovery of thermal energy from brines in a geothermal power plant, and on the recovery of energy from the exhausts of gas turbines and internal combustion engines. The present invention is concerned with the utilization of thermal energy, i.e., heat, from various heat sources, particularly heat sources having a relatively low heat content. These so-called "low grade" heat sources are found, e.g., in many industrial and commercial installations, such as refineries, chemical plants, and processing plants, as well as in geothermal sources, etc., and generally, whatever the medium, will have a temperature of or will transfer temperatures on the order of 100.degree. to 150.degree. C or 175.degree. C. The invention has application to higher temperature heat sources, of course, but the great advantage of the invention resides in the fact that low grade heat sources may be utilized.